My son (5th grade) is doing a Veggie Power project for his science fair. His question is "What produce requires the least amount (in mass) to make a battery that lasts the longest?".His hypothesis is that the citrus produce will make the best battery. When he set up the trialsusing Lemons, Oranges and Potatoes one week ago, we saw that potatoes required the least amount (in mass) to light up a 2.6V LED.. THe trial is still going on and all the LEDs are still ON.We are waiting for LEDs to go OFFto see which one lasted longest.

We want to understand that why potatoes reqd the least amount to light up LED compare toOranges and Lemons.. We need some help to reason this out in the conclusion..Please advice.